The physics of Newton's Cradle is considerably more complex than suggested
in many physics textbooks. Observation of the experiment shows
that if one ball is pulled aside and released, it is not precisely true
that only one ball is ejected from the other side. Merely some
misalignment in manufacture? It turns out that the cause is not to be
found there, but rather in the finite compressibility of the balls, which
renders collisions non-instantaneous. A compressive elastic pulse of
finite width is propagated down the line. When it ejects the last ball,
the previous two are still slightly compressed together, so they fly apart
too in a clearly visible displacement (the remaining displacements are not
usually discernible). Recognizing this, we see that the subsequent motion
cannot long conform to what is expected (and taught by many).

In our paper Rocking Newton's Cradle we present a
detailed analysis of Newton's Cradle,
based on theory, experiments, and simulations which also illustrate the
role of dissipation.


Computer simulation of Newton's Cradle which incorporates finite
 compressibility of the balls, visco-elastic dissipation due to ball-ball
collisions, and viscous drag of air.

 

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